As electronic devices such as portable information terminals have become light, thin and small in recent years, hybrid integrated circuit devices (so-called hybrid ICs) have been used in abundance in such products. In each hybrid integrated circuit device, electronic components, such as capacitors, transistors, transformers, and ICs, forming one circuit block, such as DC-DC converters, are mounted on one high density substrate by using the surface mounting method.
As a matter of course, individual electronic components of hybrid integrated circuit devices must be very small in size and low in height. Specifically, it is necessary to decrease the height of coil components (such as transformers, filters, choke coils), which tend to have relatively large height dimensions. As a result, a laid core type coil component 10, as shown in FIG. 6, has been devised.
This laid core type coil component 10 consists of a ferrite core 5, which has a flattened-columnar wound core portion (represented by a broken line) 2 and collars 3 and 4, formed integrally with the wound core portion 2, on both ends of the longitudinal direction of the wound core portion 2; a plurality of external electrodes 6 provided on end faces of the collars 3 and 4 of the ferrite core 5; and a winding 8 composed of an insulation covered conductor (such as a polyurethane covered conductor or a polyester covered conductor) wound around the winding core portion 2 of the ferrite core 5 and conductively bonded to the external electrodes 6 of the collars 3 and 4, respectively, at ends thereof, by thermo-compression bonding or the like.
In general, spacing between the collars 3 and 4 located at both ends is relatively wide, and a winding portion 9, composed of wound core portion 2 and winding 8, is positioned extremely close to a substrate. Such a configuration can be seen in FIG. 7, a side view of the laid core type coil compound 10 shown in FIG. 6 which further shows the previously described state of mounting on the substrate, as well as the magnetic flux B (i.e., magnetic lines of force) which terminate at collars 3 and 4, and emanate above and below the winding portion 9.
Various materials may be utilized for formation of the substrate 11, used for mounting of the hybrid integrated circuit device, such as alumina or glass epoxy. Typically, electrode lands 14 and a wiring pattern 12 are provided on one main surface of the substrate 11 (obverse of the substrate), and the laid core type coil component 10 and other chip components are surface mounted on the substrate by solder reflow or the like. In addition, a conductor pattern including a ground pattern 13 is provided on the opposite surface of the substrate, or in the interior of the substrate. Furthermore, multilayer substrates each having a number of conductor patterns, such as wiring patterns and ground patterns, formed within the substrate as a laminate, are frequently used. The conductor material for the conductor patterns, such as the wiring pattern 12 and the ground pattern 13, is usually aluminum, copper foil, silver foil, gold foil, silver-palladium, or the like.
Typically, in the conventional hybrid integrated circuit device as described above, the conductor pattern including the ground pattern 13 is provided as a coating, layer or plane on the reverse of the substrate to the laid core type coil component, or in the interior of the substrate, in as wide a layer as possible, in order to intensify the ground potential as a noise countermeasure as shown in FIG. 7. Since the magnetic flux B heading from collars 3 and 4 emanates above and below the winding as described above, the laid core type coil component 10 is affected by the conductor pattern, such as the ground pattern 13, in a region located right under the winding portion 9. As a result, the inductance value of the coil component 10 is lowered by approximately 10 to 30% as compared with the inductance valve before mounting.
In addition, the inductance value of the coil component 10 also may be affected by the peripheral environment of the substrate 11 on which the coil component 10 is mounted, such as the shape of the conductor pattern located directly under the wiring portion 9, and arrangement of the peripheral electronic components. Therefore, it is difficult to accurately determine beforehand the inductance value of the coil component after final mounting upon the substrate and placement of peripheral electronic components upon the substrate.